When you want to fill a large area with color in a hurry, the Paint Bucket is the tool for you. It’s right below the Brush in the Tools panel if you have two columns of tools, or below the Smart brush if you have one column. If you click it or press its keyboard shortcut (K) and then click in your image, all the available area (either your whole image or the current selection) gets flooded with the current Foreground color. It works something like the Magic Wand: Just as the Magic Wand selects only the color you click, the Paint Bucket fills only the color you click.